. The half-tone process. A practical manual of photo-engraving in half-tone on zinc, copper, and brass. the holder may be opened and the glass swung intoits place, the front shutter being drawn, and the holderbecoming to all intents and purposes a focussingscreen (fig 15). In the larger holders a piece of groundglass is placed in the holder, and the roller shutter infront is made to wind up into the top of the holder, sothat in this case also the focussing may be done. All these methods must be regarded as more or lessmakeshifts, which are not in any way comparable forconvenience with the plan
. The half-tone process. A practical manual of photo-engraving in half-tone on zinc, copper, and brass. the holder may be opened and the glass swung intoits place, the front shutter being drawn, and the holderbecoming to all intents and purposes a focussingscreen (fig 15). In the larger holders a piece of groundglass is placed in the holder, and the roller shutter infront is made to wind up into the top of the holder, sothat in this case also the focussing may be done. All these methods must be regarded as more or lessmakeshifts, which are not in any way comparable forconvenience with the plan which has now become verypopular— in Europe at any rate—of holding the screenindependently of the dark slide in a frame which cantravel to and fro in the back of the camera, thusenabling the screen tobe racked into positionafter the dark slide isinserted. Penrose andCo. patented this ideaabout 1894, and theirsystem has been ex-tended and elaboratedso that it now formsa very perfect me-chanism for holdingand operating thescreen. The appara-tus (fig. 16) consistsessentially of aframe J Fig. 48 THE HALF-TONE PROCESS. carrying a pair of adjustable bars to hold the screen, ina similar manner to that of the dark slides already des-cribed, this frame being attached to a pair of accuratelyfitting iron slides, running in brackets fixed at eachside of the interior of the camera. The slides areoperated by levers at each side, set in motionsimultaneously by a rod, to which they are fixed,passing through the camera, and having on itsoutside end a lever for working it. This leveris sometimes provided with a micrometer screw forobtaining a slow and fine movement, and also affordinga stop against which the lever can be returned withoutre-setting. A pointer projects from the inside travellingframe and indicates its position on a scale plate out-side. This plate is usually marked off with two scales,one reading in millimetres and the other in ^ parts ofan inch. It is thus possible to s
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Keywords: ., bookcentury1900, bookdecade1900, bookidhalftoneproc, bookyear1904
